Rising demand for wood products and depletion of virgin forests has led to a search for more efficient uses of harvested timber and to the development of engineered wood products as alternatives to natural solid wood products. An early example of such an alternative product is particleboard. Particleboard is manufactured from cellulosic materials, primarily in the form of discrete particles which are combined with a resin, wax, adhesive or other suitable binder and then consolidated under heat and pressure.
More recent developments of engineered wood products include wood strand products such as oriented strand board, oriented strand lumber, parallel strand lumber. A primary difference between particleboard and wood strand products is the particle geometry used in production. The particles used to make wood strand products are generally larger and may be cut to specific dimensions or oriented in a manner to impart strength and durability, thereby more closely mimicking the mechanical properties and appearance of natural solid wood. In addition, there are a number of other significant differences between the manufacturing processes, materials, recipes, and formulations used to make particleboard and those used to make wood strand products.
A known process for making engineered wood products is depicted schematically in FIG. 1. Although this process may generally apply to both particleboard and wood strand products, there are significant manufacturing, formulation, compositional and process differences between the two products. Referring to FIG. 1, wooden logs are cut into smaller wood elements as depicted by the schematic step 102. The size of the wood elements may vary depending on whether particleboard or a wood strand product is being produced. For example, wood elements suitable for particleboard may be about 0.125 inches in diameter, whereas suitable wood elements for wood strand products may be approximately 0.75 inches to 1 inch wide, 3 inches to 12 inches long, and 0.025 inches to 0.050 inches thick. A mixing device is used to apply a resin, adhesive, or another suitable binder to the wood elements as depicted in schematic step 106. After the binder is applied, the wood elements are formed into a mat as shown in schematic step 108 and the mat is consolidated under heat and pressure as shown in schematic step 110. In most processes, the wood elements are subjected to a drying step (e.g., schematic step 104) at some point prior to the consolidation to ensure the proper moisture content (schematic step 108).
One significant difference between manufacturing particleboard and manufacturing wood strand products occurs in schematic step 106. In particleboard manufacturing, a screw-type blender is used often in schematic step 106. Particleboard blenders are typically tubes which are about 1 foot to about 3 feet in diameter and about 8 feet to about 15 feet long. The wood elements are moved through the tube by a screw, and injection nozzles or spray tips located at various locations along a spinning screw shaft within the tube are used to apply the binder. The friction resulting from the movement of the screw generates heat; therefore, large chilling units are often required to keep the blender cool. A chilled metal shell may also be used to prevent adhesive build-up and clogging of the tune due to moisture condensation on the tube wall.
In wood strand product manufacturing, a drum-type blender is used in schematic step 106. Because the particles used to make wood strand products are significantly larger and have a different geometry than the particles used to make particle board, a screw-type blender would likely shred and tear the strands used for wood strand products. Wood strand product blenders are rotating drums which are about 8 feet to about 12 feet in diameter and about 25 feet to about 35 feet in length. Spinning/rotating atomizers, spray tips, nozzles or other application devices suspended along the drum axis apply the binder to the wood elements which are tumbled by the movement of the drum. Examples of such devices are provided in U.S. Pat. No. 5,914,153, which is hereby incorporated by reference. The tumbling action created by the drum's rotation increases the likelihood that a strand will pass by the application device thereby enhancing binder distribution among the wood elements. Generally no chilling equipment is required during the binder application process in a wood strand product application.
The binders used to make wood strand products are generally supplied to manufacturers as small molecules, oligomers, or relatively low molecular weight resins, which are not capable of supporting substantial loads or stresses without further polymerization. Conventional binders may be applied to the wood elements in the form of water-based liquid solutions, non-aqueous liquids, or powders. The term “curing” is used to describe the conversion of the many relatively small molecules into fewer larger, cross-linked polymer molecules that often exist as networks and are capable of resisting applied loads. This conversion process is dependent upon the ability of a substantial number of the relatively small molecules to form covalent bonds with at least two (and preferably more) other small molecules. The rate at which these covalent bonds are formed must be relatively fast in order to accommodate most commercial applications. Prior to the curing process there is typically some level of penetration or absorption of the binder into the wooden particles. Powdered binders that are used to make wood composites actually melt when they initially heated. A portion of the molten binder absorbs into the wood and continued heating of these resins causes curing. When two wooden particles are held together with external forces or pressure, and a layer of wet binder exists at the interface between the two particles, and a portion of the binder has absorbed or penetrated some sufficient depth into each of the wood particles, then the curing action of the binder results in a mechanical connection (or “bond”) between the two particles. These bonds allow structural loads to be effectively transferred from one particle to another within a wood composite product.
The binders used to manufacture of wood strand products significantly impact the properties of the resulting product. Suitable binders generally include phenol formaldehyde binders, urea formaldehyde binders, polymeric diphenylmethane diisocyanate (pMDI), MDI, and others. Many factors are involved in adhesive selection for a particular application. One significant issue wood strand product manufacturers face is cost. pMDI and MDI are significantly more expensive than urea formaldehyde binders and phenol formaldehyde binders. In situations where phenol formaldehyde binders are used, urea is often added to lower the free formaldehyde content and/or to decrease the viscosity. This addition increases the overall cost of the binder.
Another factor manufacturers must consider is the impact of the adhesive on the blender equipment. Phenol formaldehyde binders may cause build-up in the blender; whereas pMDI and MDI cause relatively less build-up when compared to phenol formaldehyde binders. This is because, among other reasons, in pMDI/MDI applications have a lower pre-press tack and a higher particle to binder ration when compared to other phenol formaldehyde binders. The formation of build-up may require shut down of equipment for clean up, maintenance, or replacement of parts; therefore, the binder's effect on the blending equipment can have a significant impact on overall cost and efficiency of production.
Because engineered wood products are often intended to function in place of natural solid wood, manufacturing products with mechanical properties close to that of natural wood is highly desirable. In most cases it is desirable to maximize the strength of the bonds between the wood particles, which tends to increase the strength of wood product up to the limit of the inherent strength of the wooden particles. One known solution to strengthen the internal bonds between the particles in wood products is to add more binder in the manufacturing process. Simply adding more binder helps increase the coverage on the wood elements, thereby increasing the internal bond are and strength between the particles. One drawback of this solution is that binders are expensive and adding more binder also adds significant costs to the manufacturing process.
U.S. Pat. No. 5,324,590 discloses a particleboard produced by coating particles of wood furnish with an adhesive comprising a foamed mixture of 96-98% urea formaldehyde and 2-4% by weight dried animal blood. The mixture is foamed to about 5-15 times the volume of the liquid binder to provide a urea formaldehyde solids content of 45-70% by weight. A foamed binder is expected to help increase the binder coverage over the particles while at the same time reducing the overall volume of binder needed for the operation.
U.S. Pat. No. 5,324,590 discloses a foamed binder for a particleboard application, but the solution would likely not be suitable in a wood strand product application. Thus, there is a need to develop a method for making a wood strand product that provides uniform binder coverage on the particles while at the same time minimizing the binder's cost. There is also a need to develop a method for making a wood strand product that uses a foamed binder that will increase the internal bond strength of the particles in the wood strand product when compared with the use of a conventional non-foamed binder. There is also a need to develop a method for making a wood strand product that has minimal adverse effects on blending equipment and requires minimal clean up and maintenance when compared with conventional methods.